Because the problems associated with conventional lighting systems using fluorescent lamps are not always fully understood, a brief description of such systems and the nature of fluorescent lamps in particular will be considered by way of background. It should be noted that some of this discussion will be continued below after the invention has been summarized because the points raised are best explained in connection with a drawing.
A fluorescent lamp, in contrast to the incandescent lamp, is an area source rather than a point source. In terms of light output, for a given amount of electrical power, the fluorescent lamp is three or four times more efficient than the incandescent lamp. The name "fluorescent" lamp is derived from the fact that an electric arc conducting through ionized mercury vapor or gas within the lamp emits ultraviolet photons which impinge on an interior coating of phosphor that then radiates or "fluoresces" longer wave length visible light photons.
Critical to the operation referred to is the conduction of an electrical current through the mercury vapor. The volt-ampere characteristics of this conduction are determined by a number of complex phonomena which lack simple definition. As discussed hereinbelow, the current in the arc discharge region of operation will continue to increase to disastrous levels unless limited by external means. In order to provide this current limiting, devices commonly known as ballasts are employed. In general, for AC operation, inductive ballasts are used, while for DC operation, resistive ballasts are generally employed. Transistor ballasts can be also used but these are impractical for most applications as explained in more detail below. Further, and more generally, resistive ballasting requires a substantial increase in power over that required by the lamp alone and systems enploying such ballasting are highly dissipative and energy inefficient.
A further problem associated with lamps such as are being discussed is that of providing adjustment of the light level in an effective, practical way. In general, both inductive and resistive ballasts simply limit the current to a design value although, as discussed below, there are ballast circuits which are specifically designed to enable adjustment of the arc current.
Another operational problem associated with fluorescent lamps is starting the lamps. In essence, the mercury within a flourescent lamp must be ionized before conduction can occur. This can be accomplished by momentarily applying a high voltage to the electrodes. If the lamps have heated electrodes, the ionizing or starting voltage is reduced. For this reason, the more common "rapid start" lamps have cathodes which are excited by separate transformer windings. Another type of fluorescent lamp is the "pre-heat" lamp which has a switch mechanism in the ballast circuit that momentarily closes or is closed upon energization so that a current flows through the lamp cathode and the inductor. The switch then opens, and due to the stored inductive energy, a voltage transient is also generated. The voltage transient coupled with the hot cathodes causes the lamp arc to conduct. Since the preheated electrodes are not externally heated after firing, preheat lamps are designed so that once the lamp is fired the rated arc current keeps the electrodes hot enough to emit electrons and to keep deleterious material from collecting on the cathodes.
A third group of lamps are the so-called "instant start" lamps. The cathodes of these lamps are designed for cold starting and the ballast circuit simply provides a sufficiently high starting voltage to cause conduction to be initiated by what is called high field emission. Once the lamp is started the rated arc current keeps the cathodes hot enough to provide emission and to boil off any contaminating materials. It is noteworthy that neither the instant start nor preheat lamps can be dimmed because these lamps are designed to use the arc current in order to keep there cathodes at a "liveable" temperature. When these lamps are used in a dimming mode, the cathode temperature is lowered and the lamp ends are blackened by material sputtering off the cathode so that, finally, the cathode is used up and the lamp ceases to function.
A further problem associated with fluorescent lamps is that of the decline in lumen output with usage. This decline is primarily caused by wear of the phosphor. Changes in temperature will also affect the lumen output. As explained in more detail hereinbelow, because of the phosphor decay problem, lighting systems are characteristically designed to initially overlight the associated area so that sufficient minimum light is provided as the light output decreases with lamp use. This approach results in a very substantial waste of energy. This problem, and other aspects thereof, as well as other problems associated with fluorescent lamps, are also considered below.
Patents of interest in this general field include some of my earlier patents, viz., U.S. Pat. Nos. 3,422,310 (Widmayer), 3,781,598 (Widmayer), 3,876,907 (Widmayer), as well as 3,531,684 (Nuckolls), 3,609,451 (Edgerly, Jr. et al), 3,801,867 (West et al), 4,012,663 (Soileau) and 3,909,666 (Tenen) the latter of which is discussed below.